The first slate quarrying at Minllyn was from open pits before 1840. Quarrying for slate slabs continued underground with several headings but there was never a high extraction rate despite a standard gauge connection with the Mawddwy Railway at Dinas Mawddwy Station; a private branch from the Cambrian Railway at Cemmes Road. Although the line has a passenger service it was primarily intended for slate quarry traffic.
The quarry eventually closed but was reopened and re-equipped in 1872 and for a short period a workforce of over 100 produced an annual extraction of 100 tons per annum. A new larger mill with 40 machines was built on the valley floor. By 1894 the workforce had been reduced to 20 with 550 tons of slate being produced.
Production continued to decline until the quarry closed in 1925, by this time there were only 3 saws and 2 planers. Tramways throughout the quarry were an unusual gauge of 2ft 4 1/4in; today much of the tramway still remains in place both on the surface and underground.
On the opposite side of the pit is the adit into the underground workings with random headings and chambers; all workings below adit level are now flooded. It is possible to climb out through an air shaft of one of the chambers. Some plant still remains in place including several winches, climbing chains and a boiler.